The need often arises for rapid, accurate, and continuous measurement of the physical properties of hydrocarbons, such as octane, gravity, vapor pressure, etc. The antiknock quality of gasoline is one of the physical properties often measured, since it is an important gasoline performance specification. In the laboratory, octane is measured using two single-cylinder variable compression ratio engines: a research octane engine (RON) which operates at lower speed and inlet temperature, and a motor octane engine (MON), which operates at higher speed and higher inlet temperature. The two values obtained from running gasoline through these two engines are averaged to obtain an antiknock index (AKI) that is believed to be closer to the antiknock performance of gasoline in automobiles. When implemented as an online method, this engine octane method of determining the octane of gasoline requires expensive equipment, frequent maintenance, the availability of prototype fuels, and takes about 20 minutes per sample to run.
Near infrared spectrometric analysis has been used to determine indirectly the qualitative properties of various hydrocarbon samples. U.S. Pat. No. 4,800,279, issued Jan. 24, 1989 to Hieftje, et al. entitled "Methods and Devices for Near Infrared Evaluation of Physical Properties of Samples", "Prediction of Gasoline Octane Number from Near Infrared Spectral Features in the Range 660-1215 nm" by Jeffery J. Kelley, et al., Analytical Chemistry, Volume 61, Number 4, Feb. 15, 1989, pp. 31320, and "Predicting Gasoline Properties Using Near-IR spectroscopy" by Stephen J. Swarin and Charlene A. Drumm, Spectroscopy, Volume 7, number 7, Sep. 1992, all describe a method of predicting the antiknock index of gasoline using near infrared spectrometry. These methods described passing energy in the near infrared region of the electromagnetic spectrum through a sample of gasoline and measuring the wavelength of radiation absorbed by the gasoline and the amount of absorption at each wavelength. This measurement results in a spectral profile, or spectrum, which can then be compared to the spectrum of a data set of samples having known antiknock indexes.
A problem can occur, however, with using this method in an online process environment. Because the density of gasoline, or any other sample, will vary with temperature, and because the spectrometer measuring instrument readings may also vary with instrument wear and temperature, the spectrum obtained from measuring a sample under a current set of conditions may not match the spectrum obtained when the sample was measured under the previous set of conditions.
The prior art methods of solving this problem have centered around stabilizing the temperature of the spectrometer and the temperature of the sample being measured. This is difficult, however, in a field site such as a refinery, where gasoline may have been stored and transported to a test cell outside in ambient weather conditions which vary with the time of the year. When the spectrometer is not located adjacent to the sample measuring site, typically a connection is made between the spectrometer and the sample using fiber optics, which introduce other variables.
It is thus apparent that there is a need in the art for an improved method of measuring the absorption of near infrared energy by a hydrocarbon. There is further need in the art for such a method that compensates for temperature fluctuations and other variations in the measurement. The present invention meets these and other needs.